The present application claims the benefit of Japanese Patent Application No. 2000-013622 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a microscope which is capable of observing a super fine structure of a specimen, and particularly, to a microscope which is capable of obtaining an image with the maximum contrast by adjusting an aperture stop to the optimal in accordance with the degree of unevenness of the surface of the specimen.
2. Description of the Related Art
When a super fine structure of a specimen is to be observed by a microscope, the resolution xcex4 thereof can be obtained by the following expression (1):
xcex4=xcex/2NAxe2x80x83xe2x80x83(1),
where xcex is the wavelength of an illumination light of the microscope, and NA is the numerical number of the objective lens. As seen from the expression (1), in order to improve the resolution xcex4 of the microscope, it is required to reduce the wavelength xcex of the illumination light, or to enlarge the numerical aperture NA of the objective lens.
When an object to be observed is a living specimen such as a cell, if the wavelength xcex of the illumination light is reduced to be below the ultraviolet spectral range, the living specimen itself is damaged due to a photochemical reaction, or the like. For this reason, it is not advantageous to reduce the wavelength xcex of the illumination light but instead the resolution xcex4 can be enhanced by enlarging the numerical aperture NA of the objective lens.
On the other hand, if an object to be observed is an inorganic substance such as a material and the resolution xcex4 is required to be greatly enhanced, the numerical aperture NA of the objective lens is enlarged and the wavelength of the illumination light is reduced, at the same time.
For instance, in the field in which a semiconductor wafer or the like is to be observed, the scale of a fine structure, typically of an integrated circuit, is gradually reducing. Thus, a repeating periodic structure of the fine structure called a line-and-space in a semiconductor process can reach a range below 0.25 xcexcm.
Currently, in order to observe such a fine structure, a microscope using deep ultraviolet rays having the wavelength xcex of 300 nm or less as the illumination light is used. For instance, a laser continuously emitting deep ultraviolet rays having the wavelength xcex of 266 nm, which are higher harmonics, four times as high as those emitted from Nd-YAG laser, is used as a light source, and an objective lens having a high numerical aperture NA of about 0.9 is employed, so as to obtain a resolution of about 0.10 xcexcm.
In such a conventional microscope, if a flat specimen having less unevenness on a surface is to be observed, an image with large resolution xcex4 and an excellent contrast can be obtained by reducing the wavelength xcex of the illumination light and by maximizing the numerical aperture NA of the objective lens.
However, when the surface of the specimen has an unevenness in height, which is substantially equivalent to a width of the specimen in a plane direction, an image with excellent contrast can be obtained more frequently with a reduced aperture stop, which is not a problem limitedly related to a microscope using deep ultraviolet rays, but becomes particularly conspicuous with a microscope having a high resolution for observing a super fine structure by employing the deep ultraviolet rays.
FIG. 10 illustrates the above case. FIG. 10 is a cross-sectional view of a specimen 704 with the surface having unevenness (including convex portions and concave portions) in height h substantially equal to the width w thereof in the plane direction. For observing the specimen 704 by a microscope, ambient light rays of a light flux 701 having a high numerical aperture are intercepted by convex portions of a specimen (sample) 704 and fail to reach a focusing surface 703. Also, the ambient light rays of a light flux 701 are scattered on the surface of the specimen 704, so as to decrease the contrast of the image.
Accordingly, by adjusting the numerical aperture NA of the light flux 701 with an aperture stop, it is possible to enhance the contrast and the comprehensive quality of the image since, if a light flux 702, as shown in the drawing, is employed as the illumination light, there is no scattered light on the surface of the convex portions of the specimen 704.
However, a value for the numerical aperture obtaining the optimal contrast depends on a width w and a height h of the unevenness of the structure of the specimen 704. For this reason, the observer has to adjust the aperture stop through trial and error for each specimen having different width w and height h, which provides a great burden on the observer.
Moreover, in the case of a microscope having a high resolution and utilizing deep ultraviolet rays, the specimen 704 is damaged by the deep ultraviolet rays so that reducing the time for adjusting the aperture stop to the minimum is necessary.
Accordingly, an object of the present invention is to provide a microscope of high resolution for observing a super fine structure of a specimen, in which the aperture stop is set to correspond to the height of the unevenness of the surface of the specimen, so as to obtain an image with the maximum contrast in a short period of time.
In order to achieve the above object, according to one aspect of the present invention, there is provided a confocal microscope for observing, by scanning a spot light for illuminating an object to be observed through an objective lens, an image of said object to be observed, which comprises: an aperture stop adjusting the numerical aperture of the objective lens; and a control unit calculating the contrast of the image of the object in accordance with each focal position while varying the focal position of the objective lens along the direction of the optical axis to obtain the height of the unevenness of the surface of the object from a fluctuation of said contrast, thereby setting the aperture stop in such a manner that the depth of focus is substantially equal to the height of said unevenness.
According to the present invention, an image can be acquired with the optimal contrast in a short period of time since the height of the unevenness of the surface of the object is measured and the aperture stop is automatically set to have the depth of focus corresponding to the height of the unevenness of the object by using the sectioning function of the confocal microscope.
In order to achieve the above object, according to another aspect of the present invention, there is provided a wide field microscope for illuminating an object to be observed with uniform light through an objective lens to observe the image of the object, which comprises: an aperture stop adjusting the numerical aperture of the objective lens; and a control unit calculating a contrast of the image of the object while varying the aperture stop and the focal position of the object lens so as to set the aperture stop and the focal position at the positions at which the contrast becomes the maximum.
According to the present invention, since the contrast of the image of the object to be observed is calculated by varying the aperture stop and the focal position so as to automatically set the aperture stop and the focal position at the positions at which the contrast reaches the maximum, an image can be obtained with the optimal contrast: in a short period of time.
In order to achieve the above object, according to still another aspect of the present invention, there is provided a microscope for observing an image to be observed through an objective lens, which comprise: an aperture stop adjusting the numerical aperture of the objective lens; and a control unit receiving the height of the unevenness of the surface of the object so as to set the aperture stop to have the depth of focus substantially equal to the height of the unevenness.
Also, the control unit is adapted to change a set value for the aperture stop in accordance with the height of the unevenness so as to set the aperture stop and the focal position of said object lens at the positions at which the contrast of the objective image becomes the maximum.
According to the present invention, a designed value for the height of the unevenness of the object to be observed and a value of deviation from the designed value can be directly input, so as to automatically determine the depth of focus giving the optimal contrast. Accordingly, an image with the optimal contrast in a short period of time when the object has a clear profile of the unevenness and a distinct deviation such as a semiconductor device can be obtained.